DC Brushless motors are utilized in various kinds of consumer equipment and industrial equipment because of their high reliability, which is in part due to the absence in such motors of commutation brushes and rectifiers. DC brushless motors are found, for example, in small cooling fans which have low noise levels, long life and high reliability characteristics even at high speed operation.
In the past, the most common method for controlling the speed of a DC brushless fan was a speed control system which uses a frequency generator. However, these speed control systems are complicated and expensive because the use of a frequency generator requires special circuitry for detecting frequency generator signals, including high gain amplifying circuitry for amplifying the usually weak frequency generator signals. In addition, the frequency generator itself is expensive since it requires a rotor magnet having multiple magnetic poles. Thus, while DC brushless motors are themselves relatively inexpensive, systems for controlling the speed of such motors could often not be justified because of their expense.
In certain applications utilizing DC brushless motors, speed control is not a critical requirement. However, with the growing use of DC brushless motors in more and more equipment, there has developed a need for an inexpensive speed control system.
Further, in equipment such as computers and photocopying machines, the noise from a DC brushless fan motor can be annoying since such equipment is usually found in a quiet office environment. One solution to this noise problem is controlling fan motor speed relative to temperature. That is to say, if the ambient air temperature within the equipment increases so will the speed of the motor fan. However, with a reduction in temperature, there is no need for high speed operation and the fan's motor speed and corresponding noise level is reduced. Noise due to fan motor speed could also be eliminated or reduced by varying motor speed in steps as a function of the equipment's operating mode.
An objective of the present invention to provide an uncomplicated and relatively inexpensive speed control system for DC brushless fan motors that achieves beneficial speed control without the use of a frequency generator.
A further objective of the speed control system of the present invention is to maintain the speed of a DC brushless motor substantially constant even though the motor may be subjected to variations in load and/or variation in supply voltage.
Still another objective of the present invention is to provide a speed control system which varies the rotational speed of a DC brushless motor as a function of ambient air temperature. Thus, if the DC brushless motor is utilized as a fan motor, the rotational speed of the motor will increase if the ambient air temperature increases, but will decrease when there is a decrease in temperature.
These objectives, together with other advantages which will become subsequently apparent, reside in the details of construction and operation of a speed control system more fully described and claimed hereinafter, with reference to the accompanying drawings forming a part thereof.